Pima - I think i can answer your question -
basics are that navy or marine 4yr scholarship MIDN - have a summer active duty period each of the 3 summers during their 4 yrs - the first summer, after their freshman yr - all attend the same - the remaining 2 summers are very different - Marine track vs Navy track.
but to the poster
all I know is that the NROTC 1st class seem to learn their "service selection" ( which community - surface, subs, aviation, etc) much like USNA.
Typically it is late fall of the senior (1st class) yr.
The experienced I watched was the MIDN usually voices their goal sometimes they are unsure, but it is a topic discussed I assume with their Navy advisor. At least at my son's unit, they have an advisor each year. For those interested in aviation there are apitutide tests, physical requirements( fit in seat, swimming test, etc). I dont remember when, but the MIDN do fill out their service selection preferance sheet - list their choices arranged 1 thru 6 as I recall, then 2ndclass yr (junior yr) spring semester they learn their 1st class summer cruise assignment - this is a mandatory active duty cruise -this includes those that came with 4 yr scholarships as well as those contracted along the way - just before their senior year.
at VMI the MIDN were told all along that they would get that active duty assignment in their 1st choice community.
The short answer thus is the NROTC MIDN received their "service selections" this past October - middle of month. They are all expecting their official orders sometime this spring.
I hope this helps, hopefully someone will add to my generalizations based on what a parent learns from their MIDN - so I might be missing some parts. larrys mom